1. Field of the Invention
The present invention relates generally to semiconductor integrated circuits, and more particularly, to circuits and systems that use low voltage detect circuits for detecting when an external power supply drops below a predetermined minimum voltage. Other embodiments of the invention may relate to a voltage detect circuit capable of detecting alternative voltage levels and/or voltage ranges.
2. Description of the Related Art
The following descriptions and examples are given as background only.
Low voltage detect (LVD) circuits are often included in low-power or battery powered systems, such as portable electronic devices, to detect low-voltage conditions that may harm the low-power system or prevent the low-power system from operating properly. For example, a low voltage detect circuit may be used to protect the internal logic states of volatile memory (e.g., RAM, control registers, etc.) within a low-power system when a power supply voltage supplied thereto drops below a predetermined minimum voltage (e.g., when batteries become weak or removed). The LVD circuit may be used to compare the power supply voltage to the predetermined minimum voltage, and to generate a low voltage detect signal (LVDET) whenever the power supply voltage drops below the predetermined minimum value.
Previous LVD circuits were often based on comparing a resistively divided value of the power supply voltage to a stable reference voltage provided, for example, by a reference voltage generator (such as a bandgap reference circuit). A block diagram of a conventional low voltage detect circuit 100 is shown in FIG. 1 as including a resistor divider network 110, a bandgap reference circuit 120 and a differential amplifier or comparator 130. In FIG. 1, an externally supplied voltage (Vext) is divided by resistor divider network 110, and the divided voltage (Vdiv) is compared by comparator 130 with the stable reference voltage generated by reference voltage generator 120. In most cases, the stable reference voltage may be generated by a bandgap reference circuit, and thus, is shown here as a bandgap reference voltage (Vbg). Bandgap reference circuits are often used in LVD circuits for their ability to produce reference voltages with relatively high accuracy.
As shown in FIG. 1, resistor divider network 110 may include two or more resistors, such as resistors R1 and R2, which are coupled in series between the externally supplied voltage and ground. The resistors are generally ratioed so that the divided output voltage, Vdiv, is equal to the reference voltage, Vbg, when the externally supplied voltage, Vext, attains a predetermined voltage level. In some cases, LVD circuit 100 may be configured to produce a low-voltage detect signal (LVDET) whenever the externally supplied voltage (Vext) decreases, such that the divided voltage (Vdiv) drops below the predetermined voltage level set by the reference voltage (Vbg).
Although satisfactory for a number of applications in the past, the above approach is not wholly satisfactory for the latest generation of devices and systems for a number of reasons.
One significant disadvantage of the above circuit is that large resistors, e.g., in the range of 1.5 to 15 mega-ohms (MΩ) or more, may be required to reduce the standby current through the resistor divider network. However, the large size of the resistors can make it difficult to utilize the same low voltage detect circuit on a number of different Integrated Circuit (IC) chips having different layouts. In addition, relatively large amounts of standby current may still be dissipated even when large resistors are used in the resistor divider network. For example, a standby current of approximately 400 nano-amperes (nA) may be generated within resistor divider network 110 when 15 MΩ resistors are used. The relatively large standby current can make the circuit unsuitable for a number of low-power or battery powered applications, including memory used in mobile devices, such as MoBL™ SRAM (Static Random Access Memory), a commercially available product from Cypress Semiconductor of San Jose, Calif. Moreover, due to layout area constraints on the chips, it is generally not possible to increase the resistances within the resistor divider network to effectively reduce the standby current. In other words, layout constraints on a given IC chip may limit the size of the resistors that may be used in the divider network, thereby limiting the potential reduction in standby current.
Another disadvantage of LVD circuit 100 is the need for a stable reference voltage generation circuit, such as bandgap circuit 120. Bandgap circuit 120 tends to exacerbate the power dissipation problem by consuming additional amounts of current (e.g., a few hundred nA) and die area. If the bandgap circuit were eliminated, the saved current and area could be used elsewhere, thereby imparting greater flexibility to the overall system design. In addition, bandgap circuits tend to become considerably less accurate as the power supply voltage drops to within a few hundred milli-volts of the generated reference voltage. The decrease in accuracy is especially prevalent in the power supply ranges (e.g., about 0.9V to about 2.1V) at which LVD circuits typically operate.
Accordingly, a need exists for a low voltage detect circuit that eliminates the need for resistor divider networks and reference voltage generation circuits without adversely impacting the accuracy of the voltage detect circuit. In addition to providing other advantages, such a low voltage detect circuit would significantly reduce the amount of standby current and die area consumed by the circuit.